Aluminum parts are generally soft and offer poor resistance to wear unless special measures are taken to improve the surface character of the part. Anodized coatings are commonly used for this purpose and provide good protection against wear. The particular type of anodizing process used and the thickness of the coating affect the wear quality of the surface. A "hard anodized" coating is especially useful in offering wear protection to a part. It is important to be able to reliably measure the wear resistance of an anodized coating to assure its suitability for a particular use or to control the anodizing process.
One type of surface wear measurement is the pin-on-disc method which is used to determine the relative wear of two materials. A fixed pin is pressed against a test coupon of material which is rotated on a turntable about an axis spaced from the pin. Strain gages sense the lateral force on the pin to determine the friction force. A track is worn in the coupon surface by the pin and wear measurements are made after a given test time. Pin wear is determined by weight loss measurements and coupon wear is determined by microscopic measurements of the scar width and/or depth. This method has not been used for anodized coatings.
The principal wear testing methods for anodized coatings are described in the paper by R. W. Thomas, "Measurement of Hardness, Wear Index and Abrasion Resistance of Anodic Coating on Aluminum", Transactions of the Institute of Metal Finishing, 1981, Vol. 59, pp 97-104. The tests include indentation tests and abrasion tests. The indentation test is a microhardness test requiring microscope measurement of an indentation made under very low loads on a specially prepared cross-sectional sample of material. The abrasive wear tests include the Taber abraser, abrasive wheel tests, and abrasive jet tests.
The Taber abraser requires a flat sample with a central hole for mounting on a turntable. A pair of freely turning abrasive wheels contact the sample during rotation and scuff the surface. The amount of wear after a given number of test cycles is determined by weight loss measurements or thickness measurements. This test has the problem of wear debris gathering on the wheels and changing the abrasive characteristics of the wheels. The abrasive wheel tests use abrasive paper wrapped on a wheel which is moved back and forth under load across a sample and is periodically indexed to furnish fresh paper. The abrasion debris is continuously cleared from the test site. The abrasive jet method comprises entraining abrasive powder in an air stream and projecting it onto a small area of a sample. The amount of powder required to penetrate a given thickness of material is used as the wear measure.
Each of the abrasion test techniques provides some information on the wear resistance of the anodized surface although they do not necessarily agree quantitatively. Nor is each test necessarily consistent. One variable is the abrasive materials used for the tests which are variable even when they are within specifications. A common feature of these abrasion test is that they deal only with removal of particles by shear in the plane of the surface or at a small angle to the surface. There is no measurement of shear normal to the surface. According to the Thomas paper, general tests can indicate the relative merit of anodized surfaces, but local circumstances are of overriding importance and only tests which closely simulate service conditions provide the correct answers. That is, the test of a material used for a particular part should be designed to address the type of failure experienced by that type of part.
A type of wear not addressed in the above tests is that due to shear normal to the surface such as where a localized pressure crushes the hard coating. This occurs, for example, when a connecting rod ball rides on a mating spherical anodized surface of an aluminum piston and transfers a large force through a localized contact area. It is desirable to have a wear test for this phenomenon which is accurate and reproducible.